What bothers more than lack of livery editor is the fact that upgrading your car has almost zero consequences on the cars appearance.
For example you buy a new car from the dealership, you fully tune so that it now delivers two times the horsepower, you lighten it up, you install all rigidity improvements yet the exterior appearance of the car is the same (only the exhaust and the hood changes), the cockpit is the same, there is no roll-cage installed etc.
Shouldn't at least weight reduction and rigidity improvement get rid of the chrome detailing of the car, install a roll-cage, replace the leathers inside the cockpit with carbon fiber, install racing instruments, get rid of passengers seats, install bucket seats, replace more body panels with lighter materials etc? And I shouldn't even mention the ludicrous aero parts in GT-Auto:
"Huge ugly big rear wing" + "Cockpit with leather and no roll cage" = "Pimp my Ride"!
Sure there are racing modifications but very few cars have them. And they feel more like a completely new car, not like upgrading your car bit by bit and transforming it to a racing machine. In other words I find car upgrading in GT5 unsatisfying, sloppy and unrealistic and I believe the addition of a livery editor (or a number decal system) to the current system wouldn't make the car upgrading any better. Actually it would make it even more ridiculous.
But hey, quantity over quality, isn't that what GT5 is all about?